1. Field of the Invention
The present invention relates to a thermal head installed in a photo-printer, a thermal printer, or the like.
2. Description of the Related Art
A thermal head installed in a photo-printer, a thermal printer, or the like includes a heat-insulating layer composed of a material that highly insulates heat, such as glass; a plurality of heating resistors that generates heat by electrical conduction; common electrodes that are electrically connected to all the heating resistors in common; and a plurality of individual electrodes electrically connected to the heating resistors individually, all of which are provided on a substrate having an excellent heat dissipation effect. In such a thermal head, the heating resistors generate heat by way of electrical conduction via the common electrode and the individual electrodes, and the heating resistors are then brought into contact with an ink ribbon or a printing medium (heat-sensitive paper) with pressure to perform printing.
FIG. 7 shows a plan view of a known thermal head (a protective layer is not shown) having a folding structure. In the folding structure, an end of a heating resistor 105a and an end of a heating resistor 105b are connected with a U-shaped folding conductor 110 to form a print dot portion D. The other end of the heating resistor 105a is connected to an individual electrode 107, and the other end of the heating resistor 105b is connected to a common electrode 108. Each of the common electrodes 108 is connected to a common line 109 extending in the direction parallel to the arranging direction of the print dot portions D. Electricity is supplied from both ends disposed in the longitudinal direction of the common line 109. A bonding pad 111 for wire bonding is provided at the other end of each individual electrode 107, the end being opposite to the heating resistor 105a. In the known bonding pad 111, a heat-insulating layer 102 is provided on a substrate 101 and a metal thin film is provided on the heat-insulating layer 102.
A molten bonding ball is welded on the surface of the bonding pad 111. A wire 123 from the bonding ball is extended and is then welded on a bonding pad 121 of a drive unit 120. The surface of the known bonding pad 111 is flat.
In the wire bonding technology, in order to improve the bonding strength by welding between the bonding pad 111 and the bonding ball, a technology in which irregularities are formed on the surface of the bonding pad 111 has been developed (refer to Japanese Unexamined Patent Application Publication No. 5-251856). FIG. 8 shows a cross-sectional view of such a bonding pad 111.
According to the known art, the bonding pad 111 is formed on the substrate 101 having a flat surface so as to have a uniform thickness, and the surface of the bonding pad 111 is then formed by cutting with a diamond-cutter or the like. However, in this known cutting method, the bonding pad 111 and the like are formed on the substrate 101 by vacuum deposition or the like and a mechanical cutting must be then performed, resulting in an increase in the number of steps of production. In addition, a mechanical external force is applied on the bonding pad 111 during cutting, the following problems may occur: The bonding pad 111 may be peeled off, it is difficult to decrease the thickness of the bonding pad 111 because the bottom part of the bonding pad 111 after cutting has a small thickness, the periphery of the bonding pad 111 may be damaged, and the like.